A proposal that could delay the transfer of thousands of defense workers to a new office building in Alexandria is set for a vote by Thursday in the House of Representatives.

The measure, sponsored by Rep. James P. Moran Jr. (D-Va.) and included in a defense spending bill, is aimed at preventing major traffic jams on I-395 near Seminary Road in Alexandria, where the building, the Mark Center, is under construction. The bill requires the Pentagon to devise plans to ease commutes and make road improvements to ensure that the already congested area doesn't get worse.

"Under the current plan, the traffic situation will be intolerable," Moran said in a statement. "A building of this size with no access to Metro should never have been considered at this location."

The job shifts, part of the 2005 Base Realignment and Closure plan, stem from post-Sept. 11 worries that the cluster of defense operations near the Pentagon and Reagan National Airport are at risk of attack. About 20,000 defense workers are being moved to car-dependent sites along the I-95 corridor in Northern Virginia; about 6,400 of those are to go to the Mark Center. The changes will create at least two choke points along I-95: near Fort Belvoir in Fairfax County and at the Mark Center.

Moran's proposal would cap at 1,000 the number of cars allowed to park at the Mark Center. That would either delay the transfers or force the military to put in place plans to limit traffic by the September 2011 deadline for the transfers. The Mark Center will be the new headquarters for the military's Washington operations.

Pentagon officials have said they plan to have 40 percent of Mark Center employees get to work without cars, but Moran's proposal goes further, giving Congress the final say over the military's traffic-management plan. It targets traffic at six intersections and compels the Pentagon to complete construction of access roads and ramps before the parking cap can be lifted.

Pentagon officials already are working on ways to limit commuters around the Mark Center, and a preliminary proposal is expected by July. Officials have said they are looking into arranging shuttle buses, altering traffic flow on turn lanes, and encouraging carpools and telecommuting.

Many of the workers are moving from Crystal City, where there is easy access to public transit. The Mark Center is more than two miles from the closest Metro station but is served by buses.

Moran has complained for months about the BRAC transfers, saying they require cash-strapped local governments to solve a traffic mess that they did not create. An off-ramp from I-395 to the Mark Center is under consideration, but environmental concerns scuttled initial proposals, and nothing could be built by the fall 2011 deadline.

Without changes, the Virginia Department of Transportation predicts "complete gridlock" on Seminary Road and "severe congestion" between the King Street and Duke Street exits on I-395.

Moran and Maryland lawmakers won approval late last year for $300 million to help solve some of the traffic problems. The money will help fund changes near Fort Belvoir and in Bethesda near the National Naval Medical Center, which is absorbing newcomers from Walter Reed Army Medical Center in Northwest Washington.

The six intersections near the Mark Center that Moran says cannot worsen are: